Flexible Truss Frame and Method of Making the Same

ABSTRACT

A composite frame includes a plurality of truss elements covered by a cap. The truss elements are flexibly coupled with each other and with the cap. The frame flexes to conform to a contoured surface to which it is attached. Once attached, the installed frame provides the required rigidity to the structure.

BACKGROUND INFORMATION

1. Field

The present disclosure generally relates to composite structures, anddeals more particularly with a flexible truss frame for stiffeningstructures, as well as a method of making the frame.

2. Background

Structures such as airframes typically contain a number of lateralstiffening elements such as frames and ribs at regular intervals alongthe length of the structure. Conventional stiffening elements may havecharacteristics which increase their cost and complexity in someapplications. For example, each of the lateral stiffening elements in anairframe may require a unique inner mold line surface contour in orderto conform to a skin at each location along the airframe. Thus, each ofthe stiffening elements is a unique part.

Rigid stiffening elements often require shimming during the assemblyprocess to accommodate buildup of manufactured tolerances. Shimming istime consuming and may require complex geometric shim shapes which mayaffect joint strength, stiffness and/or durability. The design oflateral stiffening elements in airframes may also be complicated by theneed to span intersecting stiffeners such as longitudinal hat stiffenersand blade stiffeners. In order to accommodate these longitudinalstiffeners, the lateral frames may be provided with openings referred toas “mouseholes” next to the skin to allow passage of the longitudinalstiffeners through the frames. Mouseholes add complexity to themanufacturing process and may result in undesirable stressconcentrations in the airframe.

Accordingly, there is a need for lateral stiffening elements such asframes that have a common design but which accommodate variations insurface contours at their points of attachment, thereby reducingrecurring and nonrecurring costs. There is also a need for lateralstiffening elements that reduce or eliminate the need for shimmingduring the installation process and which have standard openings thataccommodate intersecting longitudinal stiffeners.

SUMMARY

The disclosed embodiments provide a structural frame suitable forstiffening primary and secondary structures, particularly compositestructures. The frame comprises a combination of truss elements and acontinuous cap arranged to form articulations along the length of theframe which allow the frame to flex or bend during installation. Flexingof the frame allows it to conform to local surface contours of astructure during installation, however, the frame provides requiredrigidity in all planes after it is attached to the structure. The degreeof flexibility of the frame may be readily tailored to suit therequirements a particular application. The flexibility of the frameallows it to be manufactured as a standardized common part that may beused in multiple locations, thereby significantly reducing the number ofunique parts and associated recurring and nonrecurring costs. The framemay be fastened to a contoured surface without shimming, using typicalassembly installation forces. The disclosed frame may reduce total partcount, and may reduce stress concentrations in the area of intersectinglongitudinal stiffening elements.

According to one disclosed embodiment, a composite frame is providedthat is adapted to be attached to a contoured surface. The framecomprises a plurality of truss elements flexibly coupled with eachother, and a cap extending across and joined to the truss elements. Thecap is continuous and flexible, and each of the truss elements may behat-shaped in cross section. Adjacent ones of the truss elements arejoined to each other and to the cap at a flexible node. Each of thetruss elements of the cap may be formed of a fiber reinforced syntheticresin. In one embodiment, the truss elements of the cap are formed of aunitary construction. Adjacent ones of the truss elements are spacedapart to define a gap adapted to receive a transversely extendingstiffener.

According to another disclosed embodiment, a method is provided offabricating a composite truss frame. The method comprises laying upfirst and second fiber reinforcements on the tool and placing aplurality of forming mandrels between the first and second fiberreinforcements. The method further includes infusing the fiberreinforcements with resin. The method may also comprise placing a thirdfiber reinforcement around each of the mandrels, curing the infusedreinforcements and removing the mandrels from the frame after theinfused reinforcements have been cured. The fiber reinforcements may becompacted during a resin infusion process using conventional vacuumbagging techniques.

According to a further embodiment, a method is provided of fabricatingan airframe. The method comprises providing a skin having a contouredsurface and making a flexible frame having a plurality of trusselements. The method further comprises bringing the truss elements intocontact with the contoured surface of the skin by flexing the frame, andjoining the frame to the skin. Making the frame may include laying upfiber reinforcements on the tool and infusing the reinforcements withresin, and curing the resin infused fiber reinforcements. Making theflexible frame may be performed by forming a panel by joining asubstantially flat panel member with a corrugated panel member, andcutting sections of the panel.

According to still another embodiment, a method is provided offabricating a plurality of composite truss frames, comprising forming apanel having a cap and a plurality of truss elements joined to the cap,and cutting the panel into a plurality of individual truss frames.

The features, functions, and advantages can be achieved independently invarious embodiments of the present disclosure or may be combined in yetother embodiments in which further details can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the advantageousembodiments are set forth in the appended claims. The advantageousembodiments, however, as well as a preferred mode of use, furtherobjectives and advantages thereof, will best be understood by referenceto the following detailed description of an advantageous embodiment ofthe present disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is an illustration of a perspective view of a flexible trussframe according to the disclosed embodiments.

FIG. 2 is an illustration of a view in the direction shown as “FIG. 2”in FIG. 1.

FIG. 3 is an illustration of a diagrammatic side view of the truss frameshown in FIG. 1 before being flexed.

FIG. 4 is an illustration similar to FIG. 3 but showing the truss framehaving been flexed in preparation for assembly.

FIG. 5 is an illustration similar to FIG. 4 but showing the flexed trussframe having been attached to a convex surface.

FIG. 6 is an illustration similar to FIG. 5 showing the truss framehaving been flexed in the opposite direction and attached to a concavesurface.

FIG. 7 is an illustration of a perspective, exploded view showing twotruss frames having been cut away from a panel.

FIG. 8 is an illustration of a perspective view of a section of anairframe incorporating the disclosed truss frames.

FIG. 9 is an illustration of a perspective view of the area designatedas “FIG. 9” in FIG. 8.

FIG. 10 is an illustration of a side view of a truss frame in relationto a contoured skin showing certain frame parameters.

FIG. 11 is an illustration of a side view of a truss frame of a unitaryconstruction.

FIG. 12 is an illustration of the area designated as “FIG. 12” in FIG.11.

FIG. 13 is an illustration similar to FIG. 11 but showing a truss frameconstructed from composite components.

FIG. 14 is an illustration of the area designated as “FIG. 14” in FIG.13, illustrating the inclusion of fillet noodles.

FIG. 15 is an illustration of a perspective view of a corrugatedinfusion tool used to manufacture the flexible truss frame.

FIG. 16 is an illustration of a sectional view taken along the line16-16 in FIG. 15.

FIG. 17 is an illustration similar to FIG. 15 but showing a first plyhaving been placed over the tool.

FIG. 18 is an illustration of a perspective view showing mandrels beingcovered with fiber preforms and placed in the tool.

FIG. 19 is an illustration of a perspective view of one of the preformcovered mandrels shown in FIG. 18.

FIG. 20 is an illustration similar to FIG. 18 but showing filler noodlesbeing placed in the layup.

FIG. 21 is an illustration of a perspective view of the tool in whichanother ply is being placed over the layup.

FIG. 22 is an illustration of the tool at a later stage in which avacuum bag has been installed on the tool.

FIG. 23 is an illustration similar to FIG. 22 but showing heat beingapplied to the layup and an initial flow of resin through the fiberpreforms.

FIG. 24 is an illustration of a perspective view of the tool showing thecured part being removed and cut into individual frames.

FIG. 25 is an illustration of a flow diagram of a method of installingframes on a contoured surface.

FIG. 26 is an illustration of a flow diagram of aircraft production andservice methodology.

FIG. 27 is an illustration of a block diagram of an aircraft.

DETAILED DESCRIPTION

Referring first to FIGS. 1 and 2, the disclosed embodiments relate to aflexible truss frame 50 suitable for use in primary and secondarystructures, such as the skin 70 covering an airframe (FIG. 8). As willbe discussed below in more detail, the frame 50 may be formed ofcomposite materials and is flexible along its length during theinstallation process so as to conform to local contours of the surface72 of the skin 70. The flexibility of the frame 50 allows it to bemanufactured as a common part that fits-up to differing contours of theskin surface 72. Although the frame 50 is flexible under typicalassembly forces to conform to local contours of the skin surface 72, theframe 50 provides the required stiffness and rigidity after it isattached to the skin 70. It should be noted here that while an airframeskin 70 has been disclosed for illustrative purposes, the flexible frame50 may be employed to stiffen a wide range of other primary andsecondary structures used in a variety of applications.

The truss 50 broadly comprises a continuous cap 52, and a plurality ofaligned, continuous truss elements forming a truss type architecturewhich is efficient in transferring loads and capable of providing adesired amount of lateral stiffness required to stabilize a stiffenedstructure for a given set of loads. Adjacent truss elements 54 areconnected to each other and to the cap 52 at nodes 56. The trusselements 54 carry shear loads, and the combination of the truss elements54, the continuous cap 52 and the skin 70 carry bending loads. In thedisclosed embodiments, each of the truss elements 54 has an invertedhat-shaped cross section, comprising inclined sides 62, 64, and a base60 having an outer faying surface 58 adapted to contact the skin surface72. Inclination of the side wall 62, 64 of adjacent truss elements 58forms gaps 66 between adjacent truss elements 54. As will be discussedbelow, the frame 50 may be constructed of a wide range of syntheticmaterials, including reinforced and non-reinforced thermoplastics andthermoset resins and/or flexible metals. The cap 52 and adjacent trusselements 54 may flex slightly relative to each other at the nodes 56about axes 57 such that the nodes 56 form flexible joints orarticulations 57 in the frame 50 along its length that allow the fayingsurfaces 58 of the truss elements 54 to conform to local contours on theskin surface 72 to which the frame 50 is to be attached. Moreover, theflexibility provided at each of the nodes 56 may allow some embodimentsof the frame 50 to twist about its longitudinal axis 61 in response totorsional forces applied to the frame 50 during its installation.

FIG. 3 illustrates the frame 50 in a substantially flat state prior tobeing shaped to conform to a contoured surface 72 (FIG. 2) of the skin70 or other structure. FIG. 4 shows the frame 50 being flexed 68 at thearticulations 57 using normal assembly forces in preparation for placingthe frame 50 on the skin surface 72. FIG. 5 shows the flexed frame 50having been placed on the contoured skin surface 72 and secured to theskin 70 by means of fasteners 74. In this example, the contoured skinsurface 72 is convex shaped, however the frame 50 may be flexed at thearticulations 57 in order to conform the frame 50 to a variety of simpleor complex curves and contours of the skin surface 72. The fayingsurfaces 58 of each of the truss elements 54 engages the skin surface 72in substantially face-to-face contact over substantially the entire areaof the faying surface 58. While fasteners 74 are used to join the frame50 to the skin 70 in the illustrated example, other fastening techniquesmay be employed, including bonding or co-curing in applications wherethe skin 70 is formed of composite materials.

FIG. 6 illustrates the frame 50 having been flexed in a directionopposite that shown in FIG. 4 so that the faying surfaces 58 of theframe 50 conformally engage a concave surface 72 on the skin 70.

Referring to FIG. 7, multiple truss frames 50 may be advantageouslyfabricated from a single part in the form of a composite panel 76. Thepanel 76 comprises a substantially flat panel member 77 joinedface-to-face to a corrugated panel member 79 by bonding or co-curing.Each of the panel members 77, 79 may comprise laminated plies of fiberreinforced thermoset resin, such as without limitation, a carbon fiberreinforced epoxy resin. The plies may be laid up and cured usingprepregs or resin infusion of fiber preforms. In other embodiments, thepanel 76 may be manufactured by molding the two panel members 77, 79 asa single integrated part using a suitable fiber reinforced ornon-reinforced thermoplastic. After the panel 76 has been fabricated, itmay be cut or otherwise separated along lines 78 into individual trussframes 50 having a desired width “W” and substantially identical crosssectional shapes. If desired, truss frames 50 having differing widths“W” may be cut from the same panel 76.

FIGS. 8 and 9 illustrate an airframe 80 comprising an outer skin 70supported on a plurality of longitudinally spaced truss frames 50.Longitudinally extending stiffeners 84 are attached to the skin 70 andpass through the gaps 66 (FIG. 9) between adjacent truss elements 54.

Referring to FIG. 10, the degree of flexibility of the truss frame 50prior to its installation is dependent upon the compliance and thicknesst_(cap) of the cap 52 and the thickness t_(truss) of each of the trusselements 54. The stiffness of the truss frame 50 after its attachment tothe skin 82 installation is a function of the compliance and thicknesst_(cap) of the cap 52, the thickness of the t_(skin) 70, the truss angleα alpha between the cap 52 and the sides 62, 64 of the truss elements54, as well as the width “W” of the faying surfaces 60. Importantly, thedegree of flexibility of the truss frame 50 may be tailored to suit therequirements of the particular application. This tailoring may beachieved through the selection of the number and/or orientation of theplies forming the cap 52 and/or sides 62, 64, and/or through choice ofthe geometry of the truss frame 50. Thus for example, the truss frame 50may be designed such that it has just enough flexibility to allow it toconform to the curvature of a skin 82 at a particular point on theairframe 80 while imparting the required level of rigidity to theairframe 80 at that point.

FIGS. 11 and 12 illustrate an embodiment of the truss frame 50 in whichthe cap 52 and the truss elements 54 are formed as an integratedstructure, as by molding a thermoplastic material. In this embodiment,the intersection between the cap 52 and the truss elements 54 forms anintegrated node 56 that is free of distinct elements. The absence ofdistinct elements or boundaries within the node 56 may increase thedamage tolerance of the truss frame 50 when used in primary compositestructures.

FIGS. 13 and 14 illustrate another embodiment of the truss frame 50 thatmay be fabricated using prepreg or liquid molding processes. In thisembodiment, in order to increase the damage tolerance on the frame 50fillers, sometimes referred to as fillet noodles 88 are placed betweenthe truss elements 54 and the cap 52. The noodles 88 may be covered byreturn or wrap plies 75 which overlap the cap 52 as well as the trusselements 54.

FIGS. 15-24 illustrate an infusion tool and a method of fabricating thetruss frame 50 described above, using resin infusion of fiberreinforcement preforms, hereinafter referred to as fiber reinforcements.As shown in FIG. 15, a corrugated tool 96 supported on a tool base 98includes a plurality of substantially parallel slot-like grooves 100generally corresponding in cross sectional shape to that of the trusselements 54. In FIG. 17, a layup 115 is assembled by first placing a dryor substantially dry fiber reinforcement ply stack 102 on the corrugatedtool 96 and pressing the ply stack 102 into the grooves 100. Next, asshown in FIGS. 18 and 19, a plurality of mandrels 108 are each wrappedwith a fiber reinforcement preform 104, which may comprise for exampleand without limitation, a braided fiber sleeve. The mandrels 108 have across sectional shape substantially corresponding to that of the grooves100 (FIG. 6). The wrapped mandrels shown at 110 are placed in thegrooves 100, overlying the partially formed ply stack 102.

Next, as shown in FIG. 20, the fillers 88 are individually installed inthe layup 115, between adjacent ones of the wrapped mandrels 110. Then,as shown in FIG. 21, a substantially flat dry fiber reinforcement plystack 116 is placed over the layup 115. Referring to FIG. 22, the nextstep in the fabrication process comprises installing consumables such aspeel plies, breathers, resin infusion tubes, etc. (all not shown) overthe layup 115. A vacuum bag 120 is then installed over the layup 115 andsealed to the tool base 98 using a peripheral seal 122. Although notshown in the drawings, a suitable vacuum source and source of resininfusion are each coupled with the bag 120.

Referring to FIG. 23, the next step in the process comprises applying avacuum to the bag 120 in order to compact the layup 115 while the layup115 is subjected to heat 135. Resin is infused into the layup 115 whichflows 126 through the fiber preforms 102, 104, 116 thereby impregnatingthe layup 115 with resin which is cured by the heat 135. The heat may besupplied by an oven, or other heat sources.

FIG. 24 illustrates a further step in the process in which the curedpanel 76 (see also FIG. 7) is removed or de-molded 128 from thecorrugated tool 96, and each of the mandrels 108 is removed 130 from thepanel 76. Then, as shown at 132, the panel 76 is cut into individualtruss frames 50 of the desired width.

FIG. 25 illustrates a method of fabricating a truss frame 50 andinstalling the frame 50 on a contoured surface 72, such as an airframeskin 70. At 138, a first fiber reinforcement 102 is placed on acorrugated tool 96, and at 140 a second fiber reinforcement 104 iswrapped around each of a plurality of forming mandrels 108. The wrappedmandrels 110 are placed in grooves 100 in the corrugated tool 96 at step142. Fillet noodles 88 may be installed, as required, at step 144. Atstep 146, a third fiber reinforcement 116 is placed on the corrugatedtool 96, overlying the wrapped mandrels 110 and the fillet noodles 88.At 148, a vacuum bag 120 is installed over the layup 115 and sealed tothe tool 98. At 150, the bag 120 is evacuated and resin infusion of thelayup 115 is commenced. At 152, the layup 115 is subjected to anappropriate thermal cure cycle which cures the layup 115 into a finishedpanel 76.

Following curing, the panel 76 is removed from the tool 96 and cut intoindividual truss frames 50 at step 154. At 156, during installation ofthe frames 50 on the skin 70, the truss flaying surfaces 58 of the trusselements 54 are brought into contact with the contoured surface 72 ofthe skin 70 by flexing the frame 50 along its length so as to conformthe faying surfaces to the contoured surface 72. At 158, the trusselements 50 are joined to the contour skin surface 72 by bonding,fasteners or other suitable techniques.

Referring now to FIGS. 26 and 27, embodiments of the disclosure may beemployed in the context of an aircraft manufacturing and service method160 as shown in FIG. 26 and aircraft 162 as shown in FIG. 27. Turningfirst to FIG. 26, an illustration of an aircraft manufacturing andservice method 160 is depicted in accordance with an advantageousembodiment. During pre-production, aircraft manufacturing and servicemethod 160 may include specification and design 164 of aircraft 162 inFIG. 27 and material procurement 164.

During production, component and subassembly manufacturing 168 andsystem integration 170 of aircraft 162 in FIG. 27 takes place.Thereafter, aircraft 162 in FIG. 27 may go through certification anddelivery 172 in order to be placed in service 174. While in service 174by a customer, the aircraft 162 in FIG. 27 is scheduled for routinemaintenance and service 176, which may include modification,reconfiguration, refurbishment, and other maintenance or service.

Each of the processes of aircraft manufacturing and service method 160may be performed or carried out by a system integrator, a third party,and/or an operator. In these examples, the operator may be a customer.For the purposes of this description, a system integrator may include,without limitation, any number of aircraft manufacturers andmajor-system subcontractors; a third party may include, withoutlimitation, any number of vendors, subcontractors, and suppliers; and anoperator may be an airline, a leasing company, a military entity, aservice organization, and so on.

With reference now to FIG. 27, an illustration of an aircraft 162 isdepicted in which an advantageous embodiment may be implemented. In thisexample, aircraft 162 is produced by aircraft manufacturing and servicemethod 160 in FIG. 26 and may include airframe 178 with plurality ofsystems 180 and interior 182. Examples of systems 180 include one ormore of propulsion system 184, electrical system 186, hydraulic system188, and environmental system 190. Any number of other systems may beincluded. Although an aerospace example is shown, different advantageousembodiments may be applied to other industries, such as the marine andautomotive industries.

Apparatuses and methods embodied herein may be employed during at leastone of the stages of aircraft manufacturing and service method 160 inFIG. 26. In one illustrative example, components or subassembliesproduced in component and subassembly manufacturing 168 in FIG. 26 maybe fabricated or manufactured in a manner similar to components orsubassemblies produced while aircraft 162 is in service 174 FIG. 26. Asyet another example, a number of apparatus embodiments, methodembodiments, or a combination thereof may be utilized during productionstages, such as component and subassembly manufacturing 106 and systemintegration 170 in FIG, 26. A number, when referring to items, means oneor more items. For example, a number of apparatus embodiments is one ormore apparatus embodiments. A number of apparatus embodiments, methodembodiments, or a combination thereof may be utilized while aircraft 162is in service 174 and/or during maintenance and service 176 in FIG. 26.The use of a number of the different advantageous embodiments maysubstantially expedite the assembly of and/or reduce the cost ofaircraft 162.

The description of the different advantageous embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. Further, different advantageousembodiments may provide different advantages as compared to otheradvantageous embodiments. The embodiment or embodiments selected arechosen and described in order to best explain the principles of theembodiments, the practical application, and to enable others of ordinaryskill in the art to understand the disclosure for various embodimentswith various modifications as are suited to the particular usecontemplated.

1-8. (canceled)
 9. A method of fabricating a composite truss frame,comprising: laying up first and second fiber reinforcements on a tool;placing a plurality of forming mandrels between the first and secondfiber reinforcements; and infusing the fiber reinforcements with resin,creating infused fiber reinforcements.
 10. The method of claim 9,further comprising: placing a third fiber reinforcement around each ofthe mandrels.
 11. The method of claim 10, further comprising: curing theresin infused first, second and third fiber reinforcements; and removingthe mandrels from the frame after the resin infused fiber reinforcementshave been cured.
 12. The method of claim 9, further comprising: using avacuum bag to compact the fiber reinforcements; and curing the infusedfiber reinforcements while the fiber reinforcements are being compacted.13. (canceled)
 14. A method of fabricating an airframe, comprising:providing a skin having a contoured surface; making a flexible framehaving a plurality of truss elements; bringing the truss elements intocontact with the contoured surface of the skin by flexing the frame; andjoining the frame to the skin.
 15. The method of claim 14, whereinmaking the frame includes: laying up fiber reinforcements on a tool,infusing the fiber reinforcements with resin, creating resin infusedfiber reinforcements; and curing the resin infused fiber reinforcements.16. The method of claim 15, wherein laying up fiber reinforcements onthe tool includes: placing fiber reinforcements around each of aplurality of mandrels, and placing the mandrels on the tool.
 17. Themethod of claim 15, wherein making the flexible frame includes: forminga panel by joining a substantially flat panel member with a corrugatedpanel member, and cutting sections of the panel.
 18. A method offabricating a plurality of flexible truss frames, comprising: forming apanel having a cap and a plurality of truss elements joined to the cap;and cutting the panel into a plurality of individual truss frames.
 19. Amethod of fabricating an airframe, comprising: making a corrugated panelby laying up a first fiber reinforcement on a corrugated tool, wrappinga second fiber reinforcement around each of a plurality of formingmandrels, creating wrapped mandrels, placing the wrapped mandrels in thecorrugations of the tool, placing fillet noodles in the corrugations,laying up a third fiber reinforcement over the wrapped mandrels and thefillet noodles, sealing a vacuum bag over the tool, drawing a vacuum inthe bag to compact the fiber reinforcements, infusing the first, secondand third fiber reinforcements with resin, creating resin infused fiberreinforcements, curing the resin infused fiber reinforcements, creatinga cured panel; removing the cured panel from the tool; cutting the panelinto a plurality of flexible truss frames; providing a contoured skin;and attaching the truss frames to the skin, including flexing the framesto conform to the contour of the skin.
 20. A flexible frame for use inan airframe, comprising: a flexible elongate composite cap; a pluralityof continuous composite truss elements each having an invertedhat-shaped cross section, each of the truss elements further includingsides connected to the cap at nodes forming flexible articulations, andhaving a faying surface between the sides adapted to engage a structureto which the frame is to be attached, wherein the sides of adjacent onesof the truss elements are spaced apart to form a gap adapted to receivea stiffener therethrough; and a plurality of fillet noodles between thesides the truss elements and the cap, wherein the cap and the trusselements are flexible prior to attachment to the structure but providerigidity to the structure after the frame is attached to the structure.